1. Field of the Invention
The present invention relates to an optical disc apparatus for reading information recorded on an optical disc medium such as a CD, a DVD, and a Blu-ray disc (trademark), a method of controlling the optical disc apparatus, and an information storage medium.
2. Description of the Related Art
In recent years, various optical disc media have been used as information recording media. An optical disc apparatus is used to read information recorded on such an optical disc medium. The optical disc apparatus includes an optical pickup which irradiates the optical disc medium with light to detect reflected light from the optical disc medium. The optical disc apparatus controls a drive system for relatively moving the optical pickup with respect to a surface of the optical disc medium to move the optical pickup to a position at which the information can be read. In this manner, the optical disc apparatus reads the information from the optical disc medium.
The optical disc medium may move slightly vertically due to axial runout caused by the rotation of the optical disc medium when the information is read. Moreover, a distance from the center of the optical disc medium to a track of the optical disc medium on which the information is recorded gradually changes while the information is being continuously read. Therefore, servo control is performed when the information is read. In the servo control, feedback is performed on the drive system according to an output signal output from the optical pickup to maintain a state where the information recorded on the optical disc medium is readable. As a specific example, the optical disc apparatus performs focus servo control for adjusting the distance between the optical pickup and the surface of the optical disc medium to maintain a state where the light radiated from the optical pickup is focused on a signal surface of the optical disc medium or tracking servo control for moving the optical pickup in a diameter direction of the optical disc medium, to allow a focus of the light to follow a tracking position.
When the optical disc apparatus implements the servo control, various control parameters are set for the control. For example, in the focus servo control, a parameter (focus bias parameter) regarding an offset value for a distance between an objective lens included in the optical pickup and the surface of the optical disc medium and a parameter for the correction of spherical aberration of the objective lens can be given as the parameters described above. By appropriately adjusting the control parameters, information reading accuracy of the optical pickup can be improved.
Set values to be set for the control parameters vary depending on various factors such as the type of the optical disc medium to be read, individual variability of the optical disc medium, and an environment of use of the optical pickup. Therefore, the optical disc apparatus is required to perform a parameter adjustment operation in advance before starting to read the information from the optical disc medium. Specifically, the parameter adjustment operation is control for changing the set value of the control parameter within a setting allowable range, attempting to read the information from the medium with several set values being set, evaluating the information reading accuracy at this time to calculate the set value (adjusted value) which can improve the information reading accuracy, and setting the calculated set value as the control parameter (for example, see Japanese Patent No. 4001024).
In the parameter adjustment operation as described above, an evaluation value such as, for example, a jitter value is used for evaluating the reading accuracy. The jitter value in this case indicates a time deviation of a reproduced signal obtained from an output of the optical pickup with respect to a reference clock. Such an evaluation value and the set value of the control parameter generally have a relationship which can be approximated by a quadratic curve. FIG. 19 is a graph showing an example of the relation between the set value of the control parameter and the evaluation value as described above. In FIG. 19, the set value of the control parameter is represented on an abscissa axis (X-axis), whereas the evaluation value is represented on an ordinate axis (Y-axis). The jitter value is used as the evaluation value in this case. Therefore, the reading accuracy becomes higher as the evaluation value decreases. In the example illustrated in FIG. 19, the relation between the set value of the control parameter and the evaluation value is represented by a concave upward parabola. The evaluation value can be reduced (that is, the information reading accuracy can be improved) by a value on the X-axis, which corresponds to the position in the vicinity of a vertex of the parabola, being set as the set value of the control parameter.
When the optical disc apparatus performs the parameter adjustment operation by taking advantage of the relation between the set value of the control parameter and the evaluation value as described above, it is necessary to obtain the quadratic curve which approximates the relation between the set value of the control parameter and the evaluation value in the following manner. Specifically, while the control parameter is respectively set to at least three set values which are different from each other, the evaluation values respectively corresponding to the set values are acquired. The reason why at least three sample data items are required is as follows. Unless at least three points as illustrated in FIG. 19, that is, a point Pa at which a slope of a tangent becomes negative, a point Pb at which the slope of the tangent becomes close to 0, and a point Pc at which the slope of the tangent becomes positive, are specified on the quadratic curve, the above-mentioned relation cannot be approximated with good accuracy.
For performing the parameter adjustment operation, the optical disc apparatus, for example, first sets a predetermined initial value as the control parameter, and then, attempts to acquire the evaluation values respectively corresponding to the set values while the set value is being increased (or decreased) by a predetermined step amount for each time. The setting allowable range of the value of the control parameter is set large to a certain extent in consideration of a variation in optimal set value corresponding to a target of adjustment depending on the type or the individual variability of the optical disc medium. For some types of optical disc media, the information can be read and the evaluation value can be acquired only within a limited range in the vicinity of the optimal set value, which is contained in the setting allowable range of the control parameter. In such a case, with the method of acquiring the evaluation value while sequentially increasing or decreasing the set value from the fixed initial value within the setting allowable range of the control parameter, the evaluation value is successfully obtained only after a number of attempts to acquire the evaluation value. As a result, the acquisition of the plurality of sample data items necessary for calculating the optimal set value sometimes takes a long time.
When the optimal adjusted values are to be set by the parameter adjustment operation as described above for the plurality of control parameters, it is sometimes desirable to collectively change the plurality of control parameters to simultaneously adjust the control parameters. The reason for this is as follows. In general, the information reading accuracy is represented by a function of the plurality of control parameters. Therefore, even when the individual control parameters are independently adjusted, the optimal adjusted values are not always obtained.
Fluctuations in the control parameters described above affect the servo control. For some combinations of the values of the plurality of control parameter, the servo control can no longer be continued (specifically, a servo error occurs) in some cases. As a specific example, FIG. 20 shows the range of the combination of the control parameters, in which the focus servo control can be performed normally (servo normal operation range R), for two control parameters, i.e., a parameter for the focus bias (FB parameter) and a parameter for the correction of spherical aberration (SA parameter). On this graph, an abscissa axis represents a value of the SA parameter, whereas an ordinate axis represents a value of the FB parameter. An area surrounded by a broken line represents the servo normal operation range R. Specifically, the servo control can be continued as long as the combination of the value of the SA parameter and the value of the FB parameter is within the servo normal operation range R.
When the parameter adjustment is performed by varying the values of the control parameters to evaluate the reading accuracy after the values of the control parameters are changed, the servo error occurs if the combination of the changed values of the control parameters is out of the servo normal operation range R described above. In this case, unless servo setting processing for starting the servo control (for example, in the case of the focus servo control, focus detection processing for adjusting a focus of an objective lens on a signal surface of the optical disc medium) is performed again, the information cannot be read. Therefore, when the reading accuracy is evaluated while changing the combination of the values of the control parameters, the combination of the values of the control parameters after being changed is required to fall within the servo normal operation range R.
However, even if the combination of the values of the control parameters after being changed falls within the servo normal operation range R, there is a fear that a servo error will occur in a process of varying the combination of the values of the control parameters when the amount of change for each of the control parameters, which are changed at one time, is large. A specific example of the occurrence of such a problem is indicated by an arrow in FIG. 20. The arrow indicates the process of changing the value of each of the control parameters from the combination of the values of the control parameters indicated by a point A to the combination of the values of the control parameters indicated by a point B. In the example illustrated in FIG. 20, the optical disc apparatus first changes the SA parameter from a state indicated by the point A to allow the state to transit to that indicated by a point X. Thereafter, the optical disc apparatus changes the FB parameter to allow the state to transit to that indicated by the point B. In this case, though both the points A and B fall within the servo normal operation range R to provide the combination of the values of the control parameters, which allows the focus servo control, the combination of the values of the control parameters moves out of the servo normal operation range R when the state of the combination transits to that indicated by the point X. Therefore, the servo error occurs to prevent the servo control from being continued. Therefore, for evaluating the reading accuracy in the state indicated by the point B, it is necessary to perform the servo setting processing again. As a result, the parameter adjustment requires extra time. As described above, when one of the plurality of control parameters is greatly changed in one time for adjusting the control parameters while changing the combination of the values of the plurality of control parameters, the servo error occurs though the combination of the values of the control parameters after being changed satisfies a servo control allowable condition. As a result, there is a danger of causing a time loss.